This invention relates to a beamed antenna system and in particular relates to the operation of a broadcast control channel as used in cellular radio communications systems so as to provide omni-coverage characteristics.
Cellular Radio systems are used to provide telecommunications to mobile users. In order to meet with capacity demand, within the available frequency band allocation, cellular radio systems divide a geographic area to be covered into cells. At the centre of each cell is a base station, through which the mobile stations communicate. The available communication channels are divided between the cells such that the same group of channels are reused by certain cells. The distance between the reused cells is planned such that co-channel interference is maintained at a tolerable level.
When a new cellular radio system is initially deployed, operators are often interested in maximizing the uplink (mobile station to base station) and downlink (base station to mobile station) range. The range in many systems are uplink limited due to the relatively low transmitted power levels of hand portable mobile stations. Any increase in range means that less cells are required to cover a given geographical area, hence reducing the number of base stations and associated infrastructure costs. Similarly, when a cellular radio system is mature the capacity demand can often increase, especially in cities, to a point where more smaller size cells are needed in order to meet the required capacity per unit area. Any technique which can provide additional capacity without the need for cell-splitting will again reduce the number of base station sites and associated infrastructure costs.
Typically, in a cellular communication system, an array of antennas is provided at each cellular base station site for providing communications to randomly distributed mobile stations, in a given area. Each base station site has a plurality of sectored antennas for providing a plurality of communication channels. A predetermined number of sites are used to constitute a sub-array of cells to provide a set of communication channels and channel allocations are repeated from subarray to subarray. Channels are allocated per sub-cell so as to minimize channel interference. Each antenna is thus required to subtend an arc of, typically, 60xc2x0 or 120xc2x0, depending on the number of antenna arrays employed.
The sectorised approach to the use of directional antennas is currently at its useful limit at 60xc2x0 beamwidth. The key disadvantages of this sectorised approach are: The radio transceivers at each cell site are dedicated to particular sectors which leads to significant levels of trunking inefficiencyxe2x80x94in practice this means that many more transceivers are needed at the base station site than for an omni-directional cell of the same capacity, and each sector is treated by the cellular radio network (i.e. the base station controller and mobile switches) as a separate cell. This means that as the mobile moves between sectors, a considerable interaction is required between the base station and the network to hand off the call between sectors of the same base station. This interaction, comprising signalling and processing at the base station controller and switch, represents a high overhead on the network and reduces capacity.
The antenna used at the base station site can potentially, make significant improvements to the range and capacity of a cellular radio system. Each cell has a single broadcast control channel that is assigned to a single frequency and is transmitted from the base station.
The base stations use the broadcast channel to identify themselves as a base station, either as the primary station in the area or, as an additional station to the one currently in use. Each mobile station will take signal strength measurements from all the broadcast channels it can receive. This information will be employed so that an optimum signal strength is received at all times and this data is employed for use in handover algorithms, on change of base station or sector covered by a particular base station. A good base station antenna pattern is a beam of narrow angular width, in both the azimuth and elevation planes. The narrow beam is directed at the wanted mobile and tracks the mobiles movements. This ensures that the mobile station is always connected to the most appropriate base station or setor. It is from the broadcast channel that a mobile station learns which frequency to use to contact the base station and when the base station has an incoming call for the mobile station. The broadcast channel is a fundamental element in a cellular radio system.
The use of cellular radio systems is governed by certain protocols some of which, for example the GSM protocol, require each broadcast channel to be transmitted continuously over the entirety of the cell. Such a constriction is very demanding for the rapidly emerging systems that utilise the concept of multiple narrow beams for the conveyance of traffic channels. These systems with their high gain narrow beams can give greater range coverage without requiring larger sized power amplifiers. Existing narrow beam systems can produce a nominally omni-directional broadcast channel in one of two ways: All of its beams can be transmitted simultaneously. This can result in phase problemsxe2x80x94not only with the base station, but also with neighbouring base stations. An additional omni-directional antenna can be used but the disadvantage of the omni antenna is that it has a significantly lower gain than a narrow beam antenna, and thus to cover the same range as the traffic channels the omni antenna requires a significantly higher power amplifier. WO94/11956 (South Western Bell) provides a base station antenna arrangement with a dedicated control channel antenna which sweeps the area of coverage of the cell.
The present invention seeks to overcome or reduce the problems associated with the use of omni-directional antennas as described above.
According to the present invention there is provided a radio communications base station antenna arrangement comprising a number of antenna apertures capable of forming a plurality of beams, wherein traffic and control channels share the same antenna apertures, and wherein selection means are provided capable of assigning a single control channel consecutively via one or more of these beams such that its area of coverage sweeps around the cell during a given period of time. The area of coverage can be around the whole or part of the cell. Preferably there is also provided means at the mobile station to isolate control information. Preferably signal strength measurements are conducted on burst type signals appearing at random intervals of time whereby a maximum period between bursts can be determined.
In accordance with another aspect of the invention there is provided a method of operating a radio communications base station antenna arrangement wherein the broadcast signal is provided in a narrow beam and which narrow beam sweeps around the whole or part of a cell during a given period of time, which broadcast signal is emitted through the same antenna aperture as traffic channels.